This application is based on Japanese Patent Application Nos. 2001-203833 filed Jul. 4, 2001 and 2001-241370 filed Aug. 8, 2001, the contents of which are incorporated hereinto by reference.
1. Field of the Invention
The present invention relates to a waveguide-type optical signal processing circuit, and more specifically, to a waveguide-type optical signal processing circuit spirally arranged in a lattice-form circuit having optical couplers and delay circuits alternately cascaded together.
2. Description of the Related Art
Various efforts are being made to develop a wavelength division multiplexing systems using a plurality of optical wavelengths in order to increase communication capacity. In such a wavelength division multiplexing system, a lattice-form circuit is widely used. The optical filter is used in a) a wavelength multiplexing circuit that operates in a transmitter to multiplexes optical signals with a plurality of wavelengths, b) a wavelength demuliplexing circuit that operates in a receiver to demultiplexes the signals into different ports, c) an equalizing circuit in an optical amplifier which amplifies attenuated optical signals, d) a dispersion equalizer that equalizes dispersion of a group delay, and the like.
FIG. 1 shows the configuration of a lattice-form circuit. Signal light incident on input waveguides 103 passes through an optical coupler 101a. Then, one of the signal lights has its phase delayed by a delay circuit 102a. Subsequently, the signal lights are transmitted to an optical coupler 101b, where they are multiplexed so as to interfere with each other. The lattice-form optical filter uses a cascaded configuration of optical couplers and delay circuits to repeatedly multiplex signal lights so that the lights interfere with each other, thereby processing the optical signal.
FIG. 2A shows the entire configuration of a conventional lattice-form circuit. FIG. 2B shows this configuration in further detail. The illustrated lattice-form circuit is a 4-stage lattice-form circuit composed of five optical couplers 201a, 201b, 201c, 201d, and 201e and four delay circuits 202a, 202b, 202c, and 202d. 
Provided that the optical waveguide has a radius of curvature R and a pitch S and the delay circuits have an optical path length difference xcex94L, the 1st stage delay circuit has a size of (2R+S)xc3x97(R+xcex94L/2), as shown in FIG. 2B. Accordingly, the lattice-form circuit has a size proportional to the number of stages N and which can be expressed as N(2R+S)xc3x97(2R+Lc+xcex94L), as shown in FIG. 2A. Here, Lc denotes the length of an optical coupler. That is, the circuit size increases in proportion to the number of stages according to a proportion constant substantially double the radius of curvature R. Consequently, the number of stages that can be laid out is markedly restricted.
Further, as the number of stages increases, more couplers and delay circuits are arranged at a pitch of (2R+S). As a result, the entire circuit is prone to be affected by a fabrication error in the wafer. Furthermore, if a half waveplate is inserted into a symmetrical axis of each of the arms of each delay circuit, the groove must be diced in proportion to the number of stages, and the half waveplate must be independently inserted into each arm. This operation is inefficient.
In a conventional lattice-form circuit with a large number of stages, the delay circuits are arranged at a pitch substantially double the radius of curvature R. Accordingly, a large circuit size is required, and yield may decrease in connection with the increase in circuit scale. Further, since the entire circuit is prone to be affected by a fabrication error, the characteristics of the circuit may be degraded. Furthermore, the operation of inserting half waveplates is inefficient.
On the other hand, known optical couplers include directional couplers, Y-branches, and MMIs (Multi Mode Interference couplers). The directional coupler is often used for 2 lights interferometer owing to its very small excess loss. Ideally, an optical divider/combiner used in a conventional optical circuit such as an optical switch of a 2 lights interferometer or a Mach-Zehnder interferometer is desired to have a branching ratio of 50%. For example, the optical divider/combiner described in Japanese Patent Application Laid-Open No. 11-344629 (1999) has a high yield with a small loss and a branching ratio very close to 50%.
However, the branching ratio required for the optical divider/combiner frequently used in the lattice-form circuit is not limited to 50%. If the branching ratio deviates from the desired value, the characteristics of the optical circuit such as excess loss, optical crosstalk, and dispersion value may be significantly degraded. Thus, the conventional dispersion equalizer uses tunable couplers that control the branching ratio of each optical divider/combiner after the optical circuit has been fabricated. This increases the number of steps required to fabricate the optical circuit as well as chip size and requires a power supply, a control and regulation system, and the like which are used to operate the tunable couplers. Therefore, it has been difficult to implement a practical dispersion equalizer.
As described above, the lattice-form circuit includes a large number of optical couplers, so that if each optical coupler is large, the entire optical circuit may have a large size, the yield of a single wafer may decrease to increase costs, or other problems may occur. The optical coupler is required to reduce a fluctuation in branching ratio, losses, and polarization, as well as its size.
It is thus an object of the present invention to provide a waveguide-type optical signal processing circuit which has a reduced circuit size, which is unlikely to be affected to be fabrication errors, and which enables a high yield to be achieved.
It is another object of the present invention to provide an optical coupler that uses directional couplers with a very small excess loss to reduce a variation in branching ratio due to a fabrication error and the polarization dependence of the branching ratio.
To attain these objects, the present invention provides a waveguide-type optical signal processing circuit having lattice-form circuits each comprising optical couplers and delay circuits alternately cascaded together using optical waveguides each composed of a core formed on a substrate to guide signal light and a clad arranged around the core and having a lower refractive index than the core, the delay circuit being connected to the corresponding optical coupler and composed of two arms, the lattice-form circuit comprising an input waveguide connected to the leading optical coupler and an output waveguide connected to the trailing optical coupler connected to the last delay circuit, wherein the series of lattice-form circuits are arranged in a form in which two spirals are combined together on the substrate so as not to cross each other and coupled together in the middle thereof, the optical waveguides, each of which is composed of two arms, are arranged so as to extend halfway around the respective spirals, and an end of the input waveguide which is not connected to the leading optical coupler and an end of the output waveguide which is not connected to the trailing optical coupler extend outward from the spirals.
Further, the optical couplers have a first and second optical waveguides and four directional couplers formed by arranging the first and second optical waveguides close to each other. The optical couplers are configured so as to couple together signal lights with a wavelength xcex which are incident on the first and second optical waveguides or branch a signal light with the wavelength xcex which is incident on the first or second optical waveguide. The optical couplers are configured so that for an optical path length difference xcex94L1 for the first and second optical waveguides between the first and second directional couplers with respect to the signal light incidence side, an optical path length difference xcex94L2 for the first and second optical waveguides between the second and third directional couplers with respect to the signal light incidence side, and an optical path length difference xcex94L3 for the first and second optical waveguides between the third and fourth directional couplers with respect to the signal light incidence side, xcex94L1=xe2x88x92xcex94L3=xcex/4 and |xcex94L2| less than 2xcex.
The above and other objects, effects, features and advantages of the present invention will become more apparent from the following description of embodiments thereof taken in conjunction with the accompanying drawings.